1. Field of the Invention
The present invention relates generally to an electrical capacitor and a method for its manufacture. More specifically, the invention is directed to an electrical capacitor composed of a stack of at least two dielectric plies of plastic, for example provided with regenerably thin metal coatings on at least one side. Metal-free insulating strips are present in alternation from ply to ply in the proximity of different edge sides and at a distance therefrom. The metal coatings are contacted with opposite polarity from ply to ply by metal layers applied to the side edges of the stack.
The invention is also directed to an electrical capacitor composed of a winding of at least two dielectric plies of plastic or paper provided with regenerably thin metal coatings on at least one side. Metal-free insulating strips are present in alternation from ply to ply in the proximity of different side edges and at a distance therefrom. The metal coatings are contacted with opposite polarity from ply to ply by metal layers applied to the side edges of the wound capacitor.
The invention of the present application is also directed to a method for manufacturing an electrical capacitor wherein dielectric bands of plastic or paper for example, that are metallized on one side are wound on a winding drum and then cut into individual capacitors for manufacturing a stacked capacitor. Alternately, the metallized dielectric bands are wound on a winding spindle for manufacturing a wound capacitor.
Further, the invention is directed to a method for manufacturing an electrical capacitor wherein a dielectric band is provided as a mother band having a width amounting to a multiple of a width required for individual capacitors. The mother band is first provided with metal strips extending in a longitudinal direction between which are situated metal-free strips of substantially equal width and lying substantially equally spaced from one another. The mother band is divided into individual bands along cut lines and the individual bands are wound to form capacitors.
2. Description of the Related Art
Film capacitors are disclosed, for example, in German Patent No. 17 64 548 and corresponding U.S. Pat. No. 3,614,561. Manufacturing methods for film capacitors are disclosed in German Patent No. 17 64 541 and corresponding U.S. Pat. Nos. 3,728,765 and 3,670,378.
Another type of film capacitor is disclosed in German Published Application No. 33 42 329, as well as in corresponding European Patent No. 0 144 857 and U.S. Pat. No. 4,563,724. Appertaining manufacturing methods for this type of film capacitor are also disclosed in these references.
Electrical wound capacitors and methods of manufacture have been known for many years. A special form of wound capacitor is disclosed in European Published Application No. 0 201 771 and corresponding to U.S. Pat. No. 4,639,832.
It is well known that for manufacturing electrical capacitors, plastic foils are used as dielectric plies provided with regenerably thin metal coatings situated thereon to provide a broad initial foil with longitudinal metal strips and metal-free insulating strips situated therebetween. Such broad initial foils are then cut to produce individual bands. Two cut directions for the broad band extend, first, through the center of a longitudinal metal strip and, second, through the center of the insulating strips. However, it is also known from German Published Application No. 28 31 736 to place longitudinal cuts so that the metal-free insulating strips are offset by a slight amount from the edge of the resulting individual band.
The capacitance of a film capacitor is calculated from the equation C=.epsilon..epsilon..sub.o n A/d where A=uL. In this equation, C is the capacitance, .epsilon. is the electrical constant, or relative permittivity, of the dielectric used in the capacitor which is, for example, paper or plastic foil, .epsilon..sub.o is the dielectric constant of a vacuum or permittivity of free space, n is the number of foil plies in the film capacitor lying one on top of another and having an area A. The area A represents the effective field area, i.e. the area between the metallizations or electrodes that is occupied by the electrical field. The area A is calculated as the product of length L and lateral overlap u of the two electrodes. d is the thickness of the dielectric plies or foils.
It may be seen from the above equation that the capacitance C varies inversely proportionally to the thickness d of the dielectric plies for a prescribed capacitor length L and lateral overlap u. The thickness of the dielectric plies or foils typically fluctuate on the order of magnitude of from 5 to 15%, so that variations of capacitance values of the same order of magnitude are the result in capacitors manufactured with a winding of constant length. As a rule, however, capacitors which have a narrower range of capacitance variation are desired. For example, capacitors wherein 99% of those manufactured have capacitance values that lie within a tolerance range of at most .+-.5%, and optimally in a range of only .+-.3%, from a rated or specified capacitance value.
To achieve this accuracy, it has hitherto been standard practice to calculate the actual foil thickness d in wound capacitors from a number za of turns wound on to the capacitor, the foil lengths aL which have been wound, and the spindle diameter do (which corresponds to the initial diameter of the capacitor winding). Either the ultimate number zo of windings is calculated or the foil length Lo is calculated required is calculated for this foil thickness to achieve the intended capacitance. Such figures can also be derived from a table.
It is also known to measure the capacitance of the winding as it is being formed during manufacture and to conclude the winding process when the rated capacitance value is reached.
In stacked film capacitors in which individual capactors are cut from an annular mother capacitor by sawing, it is known to achieve the correct value of capacitance by controlling and varying the cutting length during cutting into individual capacitors. The point of reference for setting the cutting length is either found from the measured capacitance of a capacitor having a known cut length which has already been cut off from the mother capacitor, or from the capacitance of the mother capacitor itself (see, for example, German Patent No. 1 764 542).
In summary, it can be stated that it is known to achieve a desired or rated capacitance by adjusting the length of the wound foils or of the finished, cut value capacitor. One problem which arises with the known methods is that, in addition to volume changes of the capactor due to changes in foil thickness, a volume change in the same direction is also added by variation in length. In other words, a capacitor that is thicker than usual due to thicker foils must also be thicker (or longer) than usual in order to compensate for the diminshed capacitance provided by the thicker foils. The reverse is true for thinner foils or plies. Such different sizes of capacitors lead to considerable production difficulties during handling and mounting, and particularly during integration of the capacitors into pots or capacitor cases.
Application of the methods which are known for producing a narrow tolerance capacitor are not applicable to capacitors of the above-mentioned European Patent No. 0 144 857 and corresponding U.S. Pat. No. 4,639,832. The length of the resulting individual capacitors are already determined during winding by treating the interrupted free edges thereof with a laser beam in accordance with the disclosure.